Write concern basics in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the effect of write concern { w: 0 }?

Consider a MongoDB insert operation with the write concern set to { w: 0 }. What will be the behavior of this operation?

A The operation returns immediately without waiting for any acknowledgment from the server.

B The operation waits for acknowledgment from the primary only.

C The operation waits for acknowledgment from all replica set members.

D The operation fails immediately with an error.

💡 Hint

Think about what w: 0 means in terms of server acknowledgment.

Write concern { w: 0 } means the client does not wait for any acknowledgment from the server. The operation is sent and the client continues immediately without confirmation.

❓ query_result

intermediate

What does write concern { w: 'majority' } guarantee?

In MongoDB, if you set the write concern to { w: 'majority' }, what does this guarantee about the write operation?

A The write is acknowledged after being written to all members, including arbiters.

B The write is acknowledged only by the primary node.

C The write is acknowledged immediately without waiting.

D The write is acknowledged after being written to a majority of voting replica set members.

💡 Hint

Consider what 'majority' means in a replica set context.

Write concern { w: 'majority' } ensures the write is acknowledged only after a majority of voting replica set members have written the data, providing stronger durability guarantees.

📝 Syntax

advanced

Identify the invalid write concern option

Which of the following write concern options is invalid and will cause a syntax or runtime error in MongoDB?

A { w: 2, wtimeout: 5000 }

B { w: -1, j: false }

C { w: 'majority', j: true }

D { w: 1, wtimeout: 0 }

💡 Hint

Check the valid range and types for the w option.

The w option must be a positive integer or the string 'majority'. Negative values like -1 are invalid and cause errors.

🧠 Conceptual

advanced

What happens if write concern timeout is reached?

If a write operation uses a write concern with a wtimeout set, and the timeout is reached before the write is acknowledged, what is the outcome?

A The write operation waits indefinitely until acknowledged.

B The write operation is aborted and rolled back automatically.

C The write operation completes but the client receives a timeout error.

D The write operation is acknowledged immediately ignoring the timeout.

💡 Hint

Think about what a timeout means for client acknowledgment.

If the wtimeout is reached before acknowledgment, the write may still complete on the server, but the client receives a timeout error indicating the acknowledgment was not received in time.

🔧 Debug

expert

Why does this write concern cause a write failure?

A MongoDB write operation uses the write concern { w: 3, wtimeout: 1000 } on a replica set with only 2 voting members. What error will occur and why?

A The operation fails with a write concern error because the requested w is higher than the number of voting members.

B The operation succeeds immediately because wtimeout is set to 1000 ms.

C The operation fails with a syntax error due to invalid wtimeout value.

D The operation ignores the write concern and writes only to the primary.

💡 Hint

Consider the relationship between w and the number of voting members in the replica set.

Write concern w cannot be greater than the number of voting members in the replica set. Requesting w: 3 on a 2-member set causes a write concern error.